Beamforming refers to a set of techniques with which to control a radiation pattern of a radio signal. One method of beamforming is to control a total antenna pattern of an antenna having several antenna elements by adjusting transmission weights of signal components radiating from each individual antenna element.
By choosing transmission weight, energy being transmitted may be directed towards a position of a receiver receiving the transmitted energy.
FIG. 1 shows a schematic antenna diagram achievable with beamforming by choosing weights of a phased array antenna. The schematic antenna diagram shows a strong adjustable main lobe and several weaker side lobes. The strong main lobe enables a high antenna gain of an antenna.
Beamforming in general is a technique that enables enhancing capacity and energy efficiency in a wireless network. By performing a beamforming operation, an antenna gain can be increased causing an increased strength of the signal being received; in effect a higher antenna directivity can cause a higher fraction of the energy being transmitted to be transferred to the receiver.
At the same time interference is spread over a smaller area, typically resulting in reduced interference levels for other users in the system. Increased signal to interference plus noise ratio (SINR) may result in higher bit-rates and higher capacity of an air interface. Higher SINR in a packet oriented system may result in shorter packet transmission times, which helps to reduce energy consumption in a system since transmitters and receivers may enter idle mode during a relatively larger ratio of time.
Beamforming with phased array antennas is useful for millimeter-wave radios, i.e. radios for transmitting and receiving waves of millimeter wavelength. These relatively short wavelengths reduce the size of individual antenna elements, which makes it possible to construct high-gain antenna arrays with a plurality of elements at reasonable size.
In contrast, if omni-directional antennas or antennas with low directivity are used at millimeter wavelengths, the resulting antenna aperture is small, which leads to a comparably lower gain and lower power transfer over the air interface to the receiver. This appears as an increased path loss at higher frequencies; indeed, the path loss measured is usually referred to as a signal attenuation between an isotropic radiator and an isotropic receiving antenna element.
In point-to-point communications, the higher path loss can be compensated for by increasing antenna aperture, either as a fixed reflective collector, a fixed antenna array or a phased antenna array.
Beamforming with phased antenna arrays also enables rapid adjustment of beam directions. Phased array antennas may be used in radio-location or radio-navigation (radar) and radio communication such as satellite communications and have also potential applicability in some point-to-multi-point fixed links and mobile communications.
In S. Wyne et al., “Beamforming Effects on Measured mm-Wave Channel Characteristics,” IEEE Trans. Wireless Communications, Vol. 10, No. 11, November 2011, it is observed that one effect of beamforming is a significant reduction of the observable delay spread of a received transmission over a wireless communication channel at 60 Giga Hertz (GHz).
FIG. 2 schematically presents a multipath radio propagation environment and a corresponding power delay profile when omni-directional antennas are used. The multi-path propagation environment comprises a line-of-sight (LOS) path as well as non-line-of-sight (NLOS) paths. The power delay profile correspondingly comprises a line-of-sight (LOS) component, but also strong non-line-of-sight (NLOS) components which contribute to an increased delay spread.
FIG. 3 schematically presents the same multipath radio propagation environment as in FIG. 2, but with directional antennas for transmission and reception. FIG. 3 also presents a power delay profile when using the directional antennas. It is seen that the contribution of non-line-of-sight (NLOS) components is reduced as compared to the contribution of NLOS components presented in FIG. 2, since the NLOS components in FIG. 3 are transmitted and/or received with weaker side lobes.
Subsequently received parts in the power delay profile are thus weaker and the root mean square (RMS) delay spread is shortened in FIG. 3 as compared to the one of FIG. 2.
With beamforming, transmitted power flux density can be confined to a small solid angle, effectively increasing the gain of the antenna in comparison to spreading that same energy over the entire sphere around the antenna. Other methods of beamforming such as eigen-beamforming aim to concentrate transmitted energy in such a way as to maximize the power flux density collected by the receiving antenna. Such eigen-beamforming is not necessarily directional, but typical solutions for antenna weighting will tend to favor signal paths with small propagation delay and low scattering. In this respect, the transmitting antenna array, the receiving antenna or receiving antenna array and the propagation environment form an equivalent channel that can be designed adaptively to improve communication performance. Generally one can say that beamforming may reduce the number of dominant components, both line-of-sight and non-line-of-sight components, and thus may reduce delay spread.
The objective of beamforming is to improve the fraction of transmitted energy along dominant signal directions, thus improving an amount of energy that may be captured within an aperture of a receiving antenna.
In today's transmissions systems, it is common to partition transmission blocks by insertion of guard intervals between the transmission blocks.
FIG. 4 is a schematic representation of a transmitted signal partitioned into transmission blocks with guard intervals inserted between the blocks.
When a transmission block is treated as a composite symbol, it is seen that a tail of an impulse response of one transmission block may interfere with a consecutive transmission block. This inter-block interference is analogous to inter-symbol interference observed in partial response communication channels, i.e. where the communication channel widens symbols and makes symbols interfere with subsequent symbols.
A guard interval may be inserted to avoid power from a transmission block spilling into a next consecutive transmission block when transmitted over a dispersive media, e.g. a wireless communication channel. If the guard interval is long enough to capture the tail of the impulse response of the radio channel being used, all transients from a first transmission block decay within the guard interval and will typically not interfere with a consecutive second transmission block. At the same time, the guard interval should not be longer than necessary since it is generates overhead as it takes more time to transmit data using longer guard intervals.
There are guard intervals of different types. In a first case, no signal is transmitted in the guard interval. In another case, the last part of a subsequent transmission block is transmitted in the guard interval. This part is also called the cyclic prefix, and is an artifice that allows the linear filter operation of the radio channel on the transmission to be equivalent to a circular convolution of the transmitted symbols and the radio channel response. In yet another case, the guard interval can be used to transmit a known signal, for instance in the form of pilot symbols. If this known signal appears before and after the transmission block, the first copy can be seen as the cyclic prefix of the transmission block together with the second copy of the known sequence.
In a wireless communications system some messages or transmission blocks are intended for a single device. In fact most data messages are of this kind. If a device position is known to a base station, transmissions may be beamformed directionally towards the device, enabling advantages of beamforming, such as transmitted energy to be directed to the device of the user and reduced interference towards devices of other users. Other methods of beamforming that may be implemented have the same goal of improving Signal-to-Interference and Noise Ratio (SI NR) at the device, and may use antenna adaptation to transfer energy towards one or more devices along more than one path, the energy being used to carry one or more streams to the plurality of devices. Each of these paths is usually understood to be the consequence of isolated clusters of scatterers in the environment that enable multipath diversity transmission. Each device in such a system will see an equivalent multiple-input multiple-output (MIMO) channel for each transmitted stream, subject to interference that is significantly reduced as compared to when antenna adaptation is not used. If the wireless communications system is designed to enable beamforming, user-specific data can be transmitted using beamforming.
Devices as used herein may comprise user devices, such as user equipments. Devices may additionally or alternatively comprise sensors and actuators in machine-type communication (MTC).
Some messages are intended for many devices, e.g. common control signaling messages. Beamforming is typically not used when the device position is not known, for example at initial access, or when common control messages are intended to a multitude of devices, which would make beamforming to each device impractical. Therefore, there is some benefit in transmitting common control messages with reduced directivity, either without beamforming or at least with much lower beamforming gain as compared to a beamforming gain used when transmitting user-specific data.
Many popular communication standards based on orthogonal frequency division multiplexing (OFDM) do allow for variation of the guard interval. Long term evolution (LTE) and Worldwide interoperability for microwave access (WiMAX) are examples of wireless standards which allow more than one length option for the cyclic prefix. For example, an LTE network may be configured with a cyclic prefix (CP) that is either 5 μs long or with an extended CP that is roughly 17 μs long. These CPs may be configured on a cell-by-cell basis over multiple cells, for example in a deployment of a cellular wireless communication system. The longer cyclic prefix may be used in rural areas in which cells tend to be much larger than in a city.
Even though user-specific data may be transmitted using beamforming and thus allowing a short guard interval to be used, since beamforming reduces delay spread, the wireless communications system may anyway have to use a longer guard interval to match the larger delay spread of transmissions without, or with less, beamforming, such as transmission of common control messages. Also, while narrow beams imply less delay spread, they also imply poorer user localization. Therefore wide beams may be needed to allow users to locate the system, but this means more delay spread. Larger delay spread needs longer guard interval. Lower delay spread can be handled with a shorter guard interval.
A wireless communications system designed to apply a long guard interval between consecutive transmission blocks would result in a system with more overhead than necessary for those communications that occur over equivalent channels having a short power delay profile.
There is hence a need for a solution addressing the issues discussed above.